dhcpv4: parse and emit DHCP Option 42 (NTP servers)

[Aditya-9-6]

Closes #1178.

Summary

This PR adds support for parsing and emitting DHCP Option 42 (Network Time Protocol Servers) as defined in RFC 2132 §8.3.

Changes

• wire::dhcpv4:
  • Defined MAX_NTP_SERVER_COUNT: usize = 3;.
  • Added ntp_servers: Option<Vec<Ipv4Address, MAX_NTP_SERVER_COUNT>> to Repr.
  • Added parsing and serialization for field::OPT_NTP_SERVERS (42).
  • Added unit test test_emit_offer_ntp.
• wire:
  • Re-exported DHCP_MAX_NTP_SERVER_COUNT.
• socket::dhcpv4:
  • Included dhcpv4_field::OPT_NTP_SERVERS in DEFAULT_PARAMETER_REQUEST_LIST.
  • Added ntp_servers to dhcpv4::Config.
  • Filtered unicast addresses and exposed ntp_servers in Config events.
  • Updated DHCP socket tests to verify Option 42 handling.

Verification

• cargo test --lib: 674 passed; 0 failed.
• cargo fmt --check: Clean.
• cargo clippy --lib --tests: Clean.

[codecov]

Codecov Report

❌ Patch coverage is 98.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 82.62%. Comparing base (efcba2e) to head (58881bf).

Files with missing lines	Patch %	Lines
src/wire/dhcpv4.rs	98.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1205      +/-   ##
==========================================
+ Coverage   82.57%   82.62%   +0.04%     
==========================================
  Files          83       83              
  Lines       26174    26249      +75     
==========================================
+ Hits        21613    21687      +74     
- Misses       4561     4562       +1     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.